The present invention relates to a process for producing ultrafine particles each coated with a thin film, and more particularly, to a process for producing ultrafine particles, each having a thin film including an elementary carbon substance and/or a carbon compound formed thereon, using a thermal plasma method.
Fine particles such as oxide fine particles, nitride fine particles, and carbide fine particles have been used in the production of sintered bodies, for example, electrical insulating materials for semiconductor substrates, printed wiring boards, and various electrically insulating parts, materials for high-hardness and high-precision machining tools such as dies and bearings, functional materials for grain boundary capacitors, humidity sensors and the like, or precision sinter molding materials, and in the production of thermal sprayed parts, for example, engine valves, of materials that are required to be wear-resistant at a high temperature, as well as in the fields of electrodes, electrolytic materials, and various catalysts for fuel cells. Use of such fine particles improves bonding strengths between different ceramics or different metals in a sintered body or thermal sprayed part, or denseness or functionality thereof.
One of the methods for producing such fine particles is a vapor-phase method. The vapor-phase method includes a chemical method that involves chemically reacting various gases or the like at high temperatures and a physical method that involves applying an electron beam or laser beam to substances to decompose or evaporate the substances so as to form fine particles.
An example of the vapor-phase method is a thermal plasma method. The thermal plasma method is a method of producing fine particles by instantaneously evaporating a raw material in thermal plasma and then quenching and condensing/solidifying the evaporated material to produce fine particles. This method has many advantages such as high cleanness, high productivity, applicability to high melting point materials because of high heat capacity at high temperatures, and easy preparation of composite material particles as compared with other vapor-phase methods. Therefore, the thermal plasma method is often used as a method of producing fine particles.
With regard to the introduction of a powdered material into a thermal plasma flame, JP 2000-219901 A (hereinafter referred to as Patent Document 1) describes a method of producing oxide coated fine metal particles, involving combining fine metal particles with a powdery raw material for a coating layer, supplying the resultant material mixture into a thermal plasma (i.e., thermal plasma flame) of an inert or reducing atmosphere to evaporate the materials to obtain a vapor-phase mixture, and then quenching the vapor-phase mixture.
Recently, it has been increasingly required that the above-mentioned various fine particles should have smaller sizes regardless of their material.
This is because a target for which the fine particles are used is required to be of a smaller size. Here, there arises a problem in that the smaller the size of the fine particles becomes, the higher the surface activity becomes, which conversely decreases the stability of the fine particles.
For example, when metals such as iron and copper are converted into fine particles, it is well known that slowly oxidizing fine particles each having a size on the order of several micrometers (μm) result in formation of an oxide film thereon. However, in a case of fine particles each having a size on the order of few nanometers (nm) to several tens nanometers (nm) (hereinafter, referred to as “ultrafine particles” in order to distinguish them from the conventionally used “fine particles” designated based on sensory distinction), oxidation occurs abruptly which may even be dangerous.
Further, when a low melting point metal such as gold and silver is formed into fine particles, it is known that the melting point of the metal decreases abruptly when the particle size is on the order of a few nanometers (nm). The particles would readily be coalesced together even when the particle size is on the order of several tens nanometers (nm), and it becomes difficult to obtain ultrafine particles that are independent of each other.
One of the methods for producing such the ultrafine particles is proposed in JP 05-043791 B (hereinafter, referred to as “Patent Document 2”).
The technique described in Patent Document 2 is to perform vacuum deposition in the presence of a reactive gas to form carbon atom layers of a uniform thickness (i.e., an ultrafine layer on the order of few atoms to several tens atoms) on the surfaces of ultrafine powder particles (as cores).